1. Field of the Invention
This invention relates generally to operational amplifiers, and more particularly, to an improved CMOS operational amplifier utilizing a push-pull output stage.
2. Description of the Prior Art
Operational amplifiers comprised entirely of MOS field-effect transistors (MOSFETs) are well known. Such amplifiers exhibit large input impedance which is characteristic of the gate-to-source impedance of MOSFETs in general. Since the MOSFET is a voltage driven device as contrasted to bipolar transistors which are current driven devices, extremely low offset and bias currents can be realized using MOSFETs for input transistors. A MOSFET operational amplifier exhibits leakage current lying in the range of picoamperes rather than in the range of nanoamperes as found in bipolar amplifiers.
A CMOS operational amplifier can be made to operate with an extremely large input common mode voltage swing. When such an amplifier operates from a single positive supply voltage, it can operate with negative input common mode voltage signals.
A further advantage of CMOS operational amplifiers is that true complimentary symmetry can be obtained since p-channel and n-channel devices can be made using the same process. Additionally, satisfactory gains are obtainable using CMOS amplifiers due to the low capacitance of the MOSFETs.
One known CMOS operational amplifier comprises a differential stage, a biasing scheme for establishing a desired current into the differential stage, and an output stage which employs a passive MOSFET load. It has been determined, however, that the transient response of the amplifier may be greatly improved if the passive load output stage is replaced with a push-pull stage. Such an operational amplifier is available from Intersil and bears part number ICL 761X. This operational amplifier, however, requires a large number of components and therefore suffers from the disadvantage of high power consumption and increased silicon area.